A centuries-old conundrum may finally have been solved, Penelope Debelle writes.

A private art historian with a passion for the Renaissance believes she has uncovered the identity of the Mona Lisa and the heartache behind her sad and mysterious smile.

Seventeen years of research, beginning in Germany, have led the Adelaide historian Maike Vogt- Luerssen to believe that the Mona Lisa is the lovesick former Duchess of Milan, Isabella of Aragon, and not the wife of a florentine silk merchant, as has been believed.

Ms Vogt-Luerssen is an amateur historian whose research has been been published in German in her book Who is Mona Lisa? In Search of her Identity.

She has also published books on women in the 15th and 16th century, everyday life in the Middle Ages and Lucrezia Borgia.

She says the clues to the Mona Lisa are in the painting itself and in other paintings, diaries and records from the time.

Apart from the wan smile and absence of jewellery, the Mona Lisa is wearing heavy, mournful garb; Isabella of Aragon's mother died the year before Leonardo da Vinci painted his most famous work. At this time the duchess was a woman of 17 years who had recently wed her handsome but dissolute husband, Gian Galeazzo II, Maria Sforza, the Duke of Milan.

Visible on the bodice of the Mona Lisa's plain brown dress are symbols of the connected rings of the house of Sforza and below them connected knots and strings representing the connection between the dynasties of Visconti and Sforza, which she had married into. Ms Vogt-Luerssen says this narrows her down to a group of eight women.

She located significant portraits of the relevant women - Caterina Sforza, who had pale reddish hair; Bona of Savoy, who was Isabella's mother-in-law; her children Anna-Maria, Angela, Ippolita, Bianca, Empress Bianca Sforza and Beatrice d'Este. The Mona Lisa portrait of Isabella of Aragon fitted this context, she said.

Ms Vogt-Luerssen believes she has found sadness and joy in the background of Isabella of Aragon that further illuminates the masterpiece.

"Why is she looking so sad? . . . She married at the end of 1488 when she came to Milan but she had a big problem. She married her cousin, a beautiful man but he was a drinker, and he had problems with impotence."

She says diarists at the time wrote of a wonderful lady, the Duchess of Milan, who was always crying because her husband beat her. But she was also close to Leonardo da Vinci, the painter at her court for 11 years, who remained a friend, and possibly more, for most of her life. Da Vinci never sold the work, although it was coveted at the time as a superior piece. He took it with him to France, where he died.

"This was a love story," she says. "But it was most difficult because she is high and mighty and Leonardo was a painter. It wasn't allowed."

The widely accepted theory of the identity of the Mona Lisa is that is it La Gioconda, the wife of a Florentine silk merchant who commissioned the painting as an act of love. She was named as the subject of the painting by the Louvre early last century.

Ms Vogt-Luerssen discounts that theory, saying the Louvre made its own guess, to appease public curiosity, based on a written description of a portrait described in 1550 by an Italian Renaissance art historian, Giorgio Vasari. "They didn't know who it was so they took the first description they could get hold of and said this could fit, but it you look closely it does not fit," she says. "Everything the Louvre says is without proof and there were always art historians who opposed this."

A comparison between the Mona Lisa and Vasari's description of a portrait of La Gioconda reveals discrepancies. His references to it as an unfinished work, the descriptions of pale red circles around her eyes, eyebrows with separate hairs delineated from the skin and a pulse beating at her throat are not evident in da Vinci's Mona Lisa.

Experts such as Chris Marshall, senior lecturer in fine art at the University of Melbourne, are unfamiliar with Ms Vogt-Luerssen's work and remain unconvinced of her views. Dr Marshall says the La Gioconda theory may still hold because Vasari wrote his description in the mid-1500s, working from memory. By then, the original of the La Gioconda portrait was already at Fontainebleau in the possession of King Francis of France.

But by using symbols employed in Renaissance portraiture to denote background and heritage, Ms Vogt-Luerssen has found what she believes are other pictures of Isabella of Aragon that connect with and reveal some physical similarity with the Mona Lisa painting.

She says a black and white Bernardino Luini painting in Washington marked "Woman Unknown" and bearing some similarity to the Mona Lisa shows Isabella holding a weasel - the symbol of her father, Alfonso of Aragon - and wearing a cross that bore the symbol of her new husband, the Duke of Milan.